Accelerated life tests are conducted on components, systems, materials, and manufacturing processes to determine their useful life in the required product or application. Their purpose is to identify and quantify the failures and failure mechanisms which cause products to wear out at the end of their useful life. Separate accelerated life tests may be conducted for each potential wear out mechanism, since the stresses which produce failures are different for each mechanism. The test results are important to ensure the component, system, material, or process will operate as intended for a predetermined lifetime.
There are numerous accelerated life tests (e.g. reliability tests, environmental stress tests, etc.) each resulting in accelerated life test data. This data may be statistically analyzed to determine, for example, a failure rate of a component, product, or system.
Some statistical analysis methods (James F. Slifker and Samuel S. Shapiro, The Johnson System: Selection and Parameter Estimation, Technometrics Vol 22, No 2, May 1980) require a judgment on the part of the user to help fit the data. This results in the possibility of a different failure rate being calculated by different users, for the same accelerated life data.